Computer-implemented method and system for machine tool damage assessment, prediction, and planning in manufacturing shop floor

ABSTRACT

A self-aware machine platform is implemented through analyzing operational data of machining tools to achieve machine tool damage assessment, prediction and planning in manufacturing shop floor. Machining processes are first identified by matching similar processes through an ICP algorithm. Machining processes are further clustered by Hotelling&#39;s T-squared statistics. Degradation of the machining tool is detected through a trend of the operational data within a cluster of machining processes by a monotonicity test, and the remaining useful life of the machining tool is predicted through a particle filter by extrapolating the trend under a first-order Markov process. In addition, process anomalies across machines are detected through a combination of outlier detection methods including SOMs, multivariate regression, and robust Mahalanobis distance. Warnings and recommendations are flexibly provided to manufacturing shop floor based on policy choice.

FIELD

This application relates in general to detecting machine tool wear,predicting machine tool remaining useful life, and providingrecommendation on machine maintenance and replacement, and inparticular, to a computer-implemented method and system for detectingmachine tool wear, predicting machine tool remaining useful life, andplanning in manufacturing shop floor.

BACKGROUND

The machining of a workpiece made from raw stock or material, such asmetal, is a form of subtractive manufacturing during which parts of theworkpiece is progressively removed until the workpiece reaches anintended shape and size. Operations during machining primarily involveturning, drilling and milling the raw material into a desired part,which respectively require removing the raw material by rotating the rawmaterial against a stationary cutting tool, axially boring holes using arotating bit with a cutting edge on a distal end whilst the workpiece isheld stationary, and cutting away the raw material using a rotating bitwith cutting edges generally along the rotating bit's sides and distalend.

Machine tools, or tools, is the parts of machines that came into contactwith the workpiece, and achieves turning, cutting, drilling, sanding,knurling, facing, deformation, or other operations. Examples of machinetools includes a drill bit or a milling cutters.

Drilling is a cutting process that uses a drill bit to cut or enlarge ahole of circular cross-section in solid materials. The drill bit is arotary cutting tool, often multipoint. The bit is pressed against theworkpiece and rotated at rates from hundreds to thousands of revolutionsper minute (RPM). This forces the cutting edge against the workpiece,cutting off chips from the hole as the workpiece is drilled.

Milling is a cutting process that uses a milling cutter to removematerial from the surface of a workpiece. The milling cutter is a rotarycutting tool, often with multiple cutting points. As opposed todrilling, where the tool is advanced along the tool's rotation axis, thecutter in milling is usually moved perpendicular to the rotation axis sothat cutting occurs on the circumference of the cutter. As the millingcutter enters the workpiece, the cutting edges of the tool repeatedlycut into and exit from the material, shaving off chips from theworkpiece with each pass.

Both drilling and milling involve traversing a rotating bit along alongitudinal axis in the Z-direction. Milling, which operates inthree-dimensional space, also involves moving the rotating bit along aplane in X- and Y-directions. The position of a machine tool at a timecan be accurately described in the 3-dimentional space. The path amachine tool traversed while processing a workpiece is referred as thetool path, and can be defined by tracking locations on X-, Y-, andZ-axes in a time window. The speed at which the workpiece advancesthrough the cutter is called feed rate, or just feed; which is mostoften measured in length of workpiece per full revolution of therotating bit.

Furthermore, there can be more than three axes to define a tool path incertain machines. In a machining process, tools may move in four or moreways to manufacture parts out of metal or other materials by millingaway excess material, by water jet cutting or by laser cutting. While atypical computer numerically controlled tools support translation inthree axes, multi-axis machines also support rotation around one or moreaxes. For example, a 5-axis machines may perform a machining processusing the linear X-, Y-, and Z-axes and two rotary axes. Typically,there are two types of 5-axis processing. The first is a 3-axisprocessing with 2-axis indexing, which involves normal linear 3-axisprocessing after indexing the inclined planes by using the two rotaryaxes. The second is processing with the two rotary axes while movingwith three linear axes.

In machine tools, a spindle is a rotating axis of the machine, whichoften has a shaft at the center of the axis. The shaft itself is calleda spindle; but also, in shop-floor practice, the word often is usedmetonymically to refer to the entire rotary unit, including not only theshaft itself, but its bearings and anything attached to the shaft. Amachine tool may have one or several spindles, such as the headstock andtailstock spindles on a bench lathe. The main spindle is usually thebiggest one. References to “the spindle” without further qualificationimply the main spindle. Spindles are powered, and impart motion to themachine tools or the workpiece. The spindle speed is the rotationalfrequency of the spindle of the machine, measured in RPM. The spindlepower is the power set to be used by the spindle. The spindle load isthe power drawn from the spindle motor. Both the positions on X-, Y-,and Z-axes and spindle speed, spindle power, and spindle load areparameters useful in defining or describing the state and history of themachine tool usage.

A critical factor in securing a successful output of a machining processis the ability of a machine tool to maintain the machine tool's accuracyand repeatability. Changes due to wear or failure of a machine tool canlead to significant losses in production and unexpected downtime. How todetect machine tool wear and tear and to predict a right time for toolmaintenance or replacement are a persistent challenge for almost allmanufacturing shops.

Monitoring systems are being developed to track operation conditions ofmachine tools; however, the data collected from sensors are not wellcorrelated with the in-process machining operating conditions, whichcompromises the prediction accuracy. As a result, the prevailing methodof manufacturing shop floor scheduling still relies on a generalizedalgorithm of time to failure (e.g. exponential fault growth). The methodlacks accuracy because the algorithm depends upon typical assumptionsrarely applicable in a real machining process. Thus a manufacturing shopwithout a reliable prediction tool always faces the dilemma of eitherbeing over-cautious by unnecessarily disrupting workflow when the toolsare still functioning well, or encountering a catastrophic loss forfailing to replace a worn tool.

Numerical control (NC) is the automation of machine tools that areoperated by precisely programmed commands encoded on a storage medium,as opposed to controlled manually via hand wheels or levers, ormechanically automated via cams alone. Most NC today is computernumerical control (CNC), in which computers play an integral part of thecontrol. The development and proliferation of CNC imposes a higherdemand for reliability of machine tools. With the advancement of sensingtechnology and automation, more information can be collected. However,independently operated machine systems and proprietary interfaces andmachine communication protocols can lead to excessive time and cost toutilize the data.

In an effort to facilitate the organized retrieval of processinformation from numerically controlled machine tools, MTConnect projectwas initiated. MTConnect is a manufacturing industry standard designedfor the exchange of data between shop floor equipment and softwareapplications used for monitoring and data analysis. However, the primaryapplications developed using MTConnect data are focused on thevisualization and reporting of Overall Equipment Effectiveness (OEE) andhistory of alarms. No insight for assessing and predicting machine toolwear and failure has been reported.

Thus there remains a need for creating a self-aware machine platform inmanufacturing shop floor, including detecting machine tool degradation,predict future failure of the machine tool, and provide advice onmanaging a manufacture floor shop, based on the data collected on themachine tool path and other parameters.

SUMMARY

A method and a system for detecting machine tool wear or anomaly,predicting machine tool remaining useful life, and planning inmanufacturing shop floor us provided. Operational data for machine toolsare collected. Tool paths are registered with the aid of a modifiedInteractive Closest Point (ICP) algorithm, which finds the best matchingbetween two clouds of 3-D spots. The result of ICP tool path matching,together with other parameters such as spindle speed, feed rate and toolnumber, are used to adaptively cluster the machining processes. For eachprocess group, a particle filter based prognostic algorithm is used topredict tool wear or spindle bearing failure. In addition, anomalies inthe background of normal behavior of the machine are detected andsecuritized to detect change in normal behavior or point to impendingfailures of the machine. Multiple machine learning, anomaly detectingalgorithms are combined in the detection to make dependable predictions.In addition, the aggregated axes traverse and the spindle load verserotating speed are incorporated to provide a measure of the wear onvarious axes in the machine. Warnings or advices are sent to managers,supervisors, or automatic machine circuit breaks to facilitatemanufacturing shop floor planning

In one embodiment, a computer-implemented system for detecting machinetool wear, predicting machine tool failure, and manufacturing shop floorplanning is provided. A data collection and parsing model collects amachine tool's operational data including positional parameters andmovement parameters during a time window, and add time stamp to thecollected data. A tool path registration model identifies a plurality ofmachining processes of the machine tool based upon a matching of thepositional parameters and the movement parameters that defines themachining processes. A process clustering module clusters the pluralityof the machining processes into one or more process clusters, based upona similarity of the machining processes. A degradation detection modeldetects the machine tool's wear by characterizing a trend of change in aparameter from the one or more clusters of machining processes performedby the machine tool. A residual life prediction module predicts themachine tool's remaining useful life by extrapolating the trend under afirst-order Markov process. The state of wear and the remaining usefullife of the machine tool can be used to help managers or maintenancecrew on manufacturing shop floor to make decisions.

In a further embodiment, a computer-implemented method for detectingdegradation and predicting failure of a machine tool is provided.Operational of a machine tool are collected. The operation data arepre-processed by adding a time stamp. A machine is identified via themachine's unique IP address. A plurality of machining processes areidentified by matching one set of spatial and movement data with anotherset of spatial and movement data to assign significant matches asbelonging to a similar machining process. Similar processes areclustered according to the similarity of the processes. Tool wear ordegradation is detected by characterizing a trend of change in aparameter from the one or more clusters of machining processes performedby the machine tool. The remaining useful life of the machine tool ispredicted by extrapolating the trend following a first-order Markovprocess and adapting a particle filter. The results of the detection canbe used for decision making on a manufacturing shop floor.

In a still further embodiment, a method for detecting anomaly of amachine is provided. A stream of operational data is collected. Thedistribution patterns of the operational data are characterized bymultiple analytical methods. New operational data is compared to thedistribution patterns of the multiple analytical methods. The presenceof one or more outliers against the distribution patterns of theanalytical methods are determined. The number of the outliers arecompared to a pre-determined threshold. Notice is sent when the numberexceeds the threshold. The analytical methods include self-organizingmaps (SOMs), multivariate regression, and robust Mahalanobis distance,respectively.

Still other embodiments of the present invention will become readilyapparent to those skilled in the art from the following detaileddescription, wherein is described embodiments of the invention by way ofillustrating the best mode contemplated for carrying out the invention.As will be realized, the invention is capable of other and differentembodiments and its several details are capable of modifications invarious obvious respects, all without departing from the spirit and thescope of the present invention. Accordingly, the drawings and detaileddescription are to be regarded as illustrative in nature and not asrestrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram showing a computer-implementedsystem for detecting degradation and predicting failure of a machinetool in accordance with one embodiment.

FIG. 2 is a flow diagram showing a computer-implemented method fordetecting degradation and predicting failure of a machine tool inaccordance with one embodiment.

FIG. 3 is flow diagram showing a computer-implemented method foridentifying a machining process based upon a match of the positionalparameters and the movement parameters that defines the processes inaccordance with one embodiment

FIG. 4 is flow diagram showing a computer-implemented method toautomatically cluster machining processes into clusters in accordancewith one embodiment.

FIG. 5 is flow diagram showing a computer-implemented method to detectthe machine tool's degradation by characterizing a trend of change of aparameter in the cluster of machining processes performed by the machinetool in accordance with one embodiment.

FIG. 6 is flow diagram showing a computer-implemented method forpredicting a cutting tool's useful remaining life in accordance with oneembodiment.

FIG. 7 is flow diagram showing a computer-implemented method fordetecting one or more outliers from the stream of the operational databy a method of self-organizing maps in accordance with one embodiment.

FIG. 8 is flow diagram showing a computer-implemented method fordetecting one or more outliers from the stream of the operational databy a method of multivariate regression in accordance with oneembodiment.

FIG. 9 is flow diagram showing a computer-implemented method fordetecting one or more outliers from the stream of the operational databy a method of robust Mahalanobis distance in accordance with oneembodiment.

DETAILED DESCRIPTION

Machine Tool Degradation Monitoring and Prediction

A self-aware system capable of integrating multiple sources ofinformation, detecting machine tool wear, and predicting failure isutilized to provide advice on manufacturing shop floor. The systemincludes components that processes operational data, automaticallyidentify machine and machining processes, clusters the machiningprocesses, detects machine tool degradation, and predicts machine toolremaining useful life. FIG. 1 is a functional block diagram showing acomputer-implemented system for detecting degradation and predictingfailure of a machine tool in accordance with one embodiment. A Web-basedserver (13) collects operational data of a machine (11) gathered througha sensor (12) installed on or close to the machine (11). An aggregate ofprograms or modules (15) are provided through a centralized server (14)that can be remotely accessed via the Web over a wide area public datacommunications network 18, such as the Internet, using wired or wirelessconnections. Alternatively, these programs or modules can be installeddirectly on the machine (11) and connected to the machine (11) throughwires (not shown). Furthermore, these programs or modules can beinstalled within the manufacturing shop that houses the machine (11) andconnected to the machine (11) through wires or wireless means (notshown). Users (19) interface with the centralized server (14) through aweb browser executing on a computer (16), hand-held device (17), orother similar devices. The central server (14) is operatively coupled toa storage device (20), within which is stored common parameters of themachine tools, and a library of machine tools, which may be specific tothe manufacturing shop. Both the server (14) and computer (16) includecomponents conventionally found in general purpose programmablecomputing devices, such as a central processing unit, memory,input/output ports, network interfaces, and non-volatile storage,although other components are possible. In addition, other computationalcomponents are possible. The programs or modules (15) includes anaggregates of modules that implements a sets of functions, includingoperational data processing, machine and tool process identification,process clustering, machine tool degradation detection, the remaininguseful life prediction, outlier detection, and manufacturing shop floorplanning analysis algorithms.

The server (14) can each include one or more modules for carrying outthe embodiments disclosed herein. The modules (15) can be implemented asa computer program or procedure written as source code in a conventionalprogramming language and is presented for execution by the centralprocessing unit as object or byte code. Alternatively, the modules couldalso be implemented in hardware, either as integrated circuitry orburned into read-only memory components, and each of the client andserver can act as a specialized computer. For instance, when the modulesare implemented as hardware, that particular hardware is specialized toperform the data quality assessment and other computers cannot be used.In another instance, the modules can be, separately or in groups,installed or integrated onto the machine and direct the machine tocollect operational data, register tool path, identify machiningprocesses, cluster the processes, detect a trend within a cluster, andpredicting remaining useful life of the machine tool. Additionally, whenthe modules are burned into read-only memory components, the computerstoring the read-only memory becomes specialized to perform the dataquality assessment that other computers cannot. The variousimplementations of the source code and object and byte codes can be heldon a computer-readable storage medium, such as a floppy disk, harddrive, digital video disk (DVD), random access memory (RAM), read-onlymemory (ROM) and similar storage mediums. Other types of modules andmodule functions are possible, as well as other physical hardwarecomponents.

A computer-implemented method is provided to provide warning orrecommendation for manufacturing shop floor supervisors or operators onaccumulated damage of the machine tools, by using automatic clusteringof machining processes and particle filter based prognostics using timeseries data of the machine tools. FIG. 2 is a flow diagram showing acomputer-implemented method for detecting degradation and predictingfailure of a machine tool in accordance with one embodiment. Operationaldata, such as a time series of positional parameters and movementparameters of a machine tool, are collected and retrieved (31). Theoperation data are pre-processed by indexing or adding a time stamp. Amachine is identified, usually by using the machine's unique IP address.A plurality of machining processes are identified, by matching one setof spatial and movement parameters with another set of spatial andmovement parameters to find close matches as belonging to one process(32). The processes thus identified are further clustered into groups orclusters according to similarity of the processes (33). Tool wear ordegradation is detected by characterizing a trend of change in aparameter from the cluster of machine processes performed by the machinetool (34). The remaining useful life of the machine tool is predicted byextrapolating the trend following a first-order Markov process andadapting a particle filter (35). Optionally, the results of thedetection can be used for decision making on a manufacturing shop floor(36).

Data Collection and Preprocessing

The machine tool operational data includes machine tool identificationparameter, positional parameters, and movement parameters. Theidentification parameter can be a tool number that identifies the tool.The positional parameter can be X-axis position, Y-axis position, andZ-axis position. For multi-axis machines, the positional parameter canfurther include the degree of rotation for the rotary axes. The movementparameter can be spindle speed, spindle speed override, feed rate, feedrate override, spindle load, X-axis load, Y-axis load, Z-axis load, andspindle power. These parameters, or variables, describe and define themachine tool's position, movement, and usage.

In one embodiment, the machine tool data may be obtained online from apublic URL for the MTConnect challenge. A query post is sentperiodically to the public URL. Returning data files are parsed; timestamps are applied to the values of the variables. A vector of a timestamp and the values of the variables are obtained. A matrix of dataindexed by multiple time stamps is stored for analysis.

Machine and Process Identification

Machine identification can be performed to localize the machine wherethe tool is installed. In one embodiment, different machines can beidentified using the IP addresses associated with the machines.

A machining process is defined as a process in which a machining toolperforms a job on a workpiece. The machining process is characterized bya combination of positional parameters and movement parameters duringthe time window of the process; i.e., by a 3-dirementional (3-D) paththe machine tools traversed during the process and the spindle speed andfeed rate associated with each location of the tool path. When themachine tool performs a different job, the tool path or the movementparameters are likely to be different, resulting a different or newmachining process. When the machine tool performs a repetitive or aclosely related job, the tool path and the movement parameters arelikely to be significantly similar, and the machining processes aretypically significantly similar.

A machining process is defined by both the positional parameters and themovement parameters. For one identified tool, the tool path consists ofmultiple X-axis, Y-axis, and Z-axis positions. These X-, Y-, andZ-positions determine the shape of the tool path in 3-D space termedshape space. For the same tool and the same process, the spindle speedsand feed rates during the process further defines the process. Thespindle speeds, feed rates, and their correlated times can berepresented by forming a 3-D shape termed movement parameter shape, orparameter shape in short, in a 3-D space termed parameter space.Together the tool path shape in the shape space and the parameter shapein the parameter space may define the vast majority of tool processes.For multi-axis machines, addition parameters, such as the degree ofrotation for a rotary axis, may be further incorporated into the shapespace and parameter space.

In one embodiment, a machining process is identified based upon asignificant similarity between both the positional parameters and themovement parameters associated with a plurality of processes using thesame tool. FIG. 3 is flow diagram showing a computer-implemented methodfor identifying a machining process based upon a match of the positionalparameters and the movement parameters that defines the processes inaccordance with one embodiment. Frist, for the plurality of themachining processes associated with the machine tool, determining a toolpath shape for each machining process in a 3-D space by plotting X-, Y-,and Z-positions of the machine tool during the each machining process(41). Second, for the plurality of the machining processes associatedwith the machine tool, determining a movement parameter shape byplotting the spindle speed, feed rate, and time of the machine tool in aparameter space for the each machining process (42). Finally,determining the match between two machining processes from the pluralityof the machining processes by matching both the tool path shapes in the3-D space and the movement parameter shapes in the parameter spacebetween the two machining processes, via finding the smallest possibledifferences between the matched shapes using an Interactive ClosestPoint (ICP) algorithm (43).

The match between the tool path shapes is determined by matching toolpath shapes using an ICP algorithm. In addition, the match between themovement parameter shapes is also determined by matching the movementparameter shapes using the ICP algorithm. In a furthe embodiment, thebest maching was found by assinging an original cloud of 3-D spacepoints as source, a transformed cloud of points as tranform, and atargeted cloud of points as target. The operation matrix of rotation,scaling and translation are T, b and c, respectively. After theoperation the following equation is obtained:transform=b*source*T+c   (1)

The ICP algorithm optimizes the operation matrix of T, b and c so thatthe difference (denoted as d) between tranform and target is minimized.The difference shows the extent to which source and target aredifferent. The smaller the difference, the better the match/overlapbetween source and target. The difference between the tool path shapesin shape spaces is denoted as d_(s), and the difference between themovement parameter shapes in the parameter spaces is denoted as d_(p).The matching measure is denoted as d_(a)=[d_(s),d_(p)] and calcualted bycombining d_(s) and d_(p).

Process Clustering

Process clustering refers to grouping, or clustering, similar machiningprocesses in a group or cluster. When machines are programmed to performdifferent jobs, different machining processes are recorded; often,machine are programmed to perform the same or repetitive job, as aresult similar machining processes are recorded. Clustering a processallows detecting a trend of the process, monitoring outliers, asdescribed supra. FIG. 4 is flow diagram showing a computer-implementedmethod to automatically cluster machining processes into clusters inaccordance with one embodiment. An initial cluster of the processes isobtained (51). The initial cluster of the processes can be obtainedthrough assigning known repetitive processes into a cluster, orborrowing an existing cluster of processes from a prior record oranalyses. At least one additional process is obtained (52). For eachadditional process, the similarity to the initial group of the processesis ascertained by determining the Hotelling's T-squared or T2 statisticsfor the matching measure of the additional process by a formula ofT2=(d _(a) −{circumflex over (d)} _(a))*s ⁻¹*(d _(a) −{circumflex over(d)} _(a))′  (2)

wherein d_(a) is the matching measure, d_(a)=[d_(s),d_(p)], d_(s) is thedifference between tool path shapes in the shape spaces, d_(p) is thedifference between movement parameter shapes in the parameter spaces,{circumflex over (d)}_(a) is the mean value of the matching measure ofthe initial cluster of the processes, and s is the covariance (53).

The T2 control limit is calculated by a formula of

$\begin{matrix}{{T\; 2_{limit}} = {\frac{\left( {N - 1} \right)\left( {N + 1} \right)p}{N\left( {N - p} \right)}{F_{\alpha}\left( {p,{N - p}} \right)}}} & (3)\end{matrix}$

where F_(α)(p,N−p) is the 100α% confidence level of F−distribution withp and N−p degrees of freedom (54).

The T2 statistics value is compared to the T2_(limit) 54. If the T2statistics is below the T2_(limit), the process is assigned to theinitial cluster of the processes (55). The parameters of the additionalprocess is used to update the centroid of the initial cluster (56). Ifthe T2 statistics is above the T2_(limit), the additional process keptout of the cluster (57).

Degradation Detection

Machine tool wear, or degradation, refers to a gradual loss ofstructural or functional integrity of the machine tool due to usage,fatigue, wear, ageing or other causes. Degradation of a machine tool maybe accompanied by a consistent trend of changes in one or moreparameters in a cluster of machining processes performed by the machinetool. Detecting this trend above signal noise may enable the detectionof underlying degradation of the machine.

In one embodiment, the trend of the change in a parameter in a clusterof machining processes performed by the machine tool is detected by amonotonicity test. FIG. 5 is flow diagram showing a computer-implementedmethod to detect the machine tool's degradation by characterizing atrend of change of a parameter in the cluster of machining processesperformed by the machine tool in accordance with one embodiment. Acluster of the processes are obtained (61). The values of a parameter ofthe machine tool performing the cluster of the processes are obtained(62). The trend of the parameter is characterized through aunidirectional change of the parameter via a monotonicity test asdescribed in

$\begin{matrix}{{{Monotonicity}(F)} = {\frac{{\#{d/{dF}}} > 0}{n - 1} - \frac{{\#{d/{dF}}} < 0}{n - 1}}} & (4)\end{matrix}$

where F is the measurement of the parameter, n is the number ofmeasurement in a period of time. F represents a parameter or feature andd/dF is the derivative (63). The maximum value of Monotonicity equals to1 only if the feature is monotonically increasing. The value ofmonotonicity indicates the increasing trend of the spindle power, whichindirectly indicates the degradation of the cutting tool. A threshold ofthe monotonicity value can be set up according to a set of factors,including the nature of the tools, the effect of tool failure onworkflows, among others (64). When the threshold is exceeded, themachine tool may be red-flagged, and a notification may be generated orsent out (65).

A number of parameters from the cluster of machining processes performedby the machine tool can selected for the purpose of revealing the trendof change through the monotonicity test. In one embodiment, spindlepower is selected. Typically, the spindle power increase is proportionalto the increased severity of tool wear for similar machining processes,because a blunted cutting tool compensate the degradation or decay ofthe cutting edge by increasing spindle power or speed. In a furtherembodiment, the machine tool's torque is selected. Typically, themachine tool's torque increase is proportional to the increased severityof tool wear for similar machining processes, because a blunted cuttingedge requires a higher level of force to perform similar jobs comparedto a non-bunted cutting edge. In a still further embodiment, heatgenerated by the machine tool during the machine tool processes isselected. Although heat generation is part of machining processes,increased heat generation for a similar machining process is oftenassociated with the loss of the structural or functional integrity ofthe tool. In a yet further embodiment, the friction of a bearingassociated with the machine tool during the machine tool process isselected. An increase of the friction level is often associated with thewear of the bearing. Other parameters are possible.

In a still yet further embodiment, multiple parameters can be selectedfor the analysis. If a degradation trend can be detected from themultiple processes belong to a same cutting tool, the predicationconfidence is increased compared to using one parameter.

Degradation Prediction

The degradation trend described supra can be extrapolated to infer theremaining useful life of the machine tool for the same or similarprocess. In one embodiment, a particle filter is adapted for theprediction. The prediction can be made using a continuous Bayesianupdate method assuming the fault growth following a physics-based systemdegradation model. In a further embodiment, a first-order Markov processis introduced into the particle filter prediction model. The first-orderMarkov process provides a more robust prediction when the current statewas only dependent upon the last state. In a still further embodiment,the first-order Markov process is further defined into a second orderpolynomial model such as:Xk=a _(k) t _(k) +b _(k) t _(k) ² +c _(k)   (5)

where X_(k) is the system state (tool wear in this case), t_(k) is thetime at step k, and a_(k),b_(k),c_(k) are the parameters of the secondorder polynomial model. The Equation(5) can also be expressed as aMarkov model as follows:Xk=X _(k−1)+(a _(k)+2bt _(k−1))Δt ₊ b _(k) Δt ²   (6)

The parameter identification and state estimation can be performed inparallel. The prediction method does not require system linearity. In ayet further embodiment, the machine tool is a cutting tool and theparameter used for prediction is the spindle power. FIG. 6 is flowdiagram showing a computer-implemented method for predicting a cuttingtool's useful remaining life. A cluster of the processes performed by acutting tool is obtained (71); the values of spindle powers of theprocesses is obtained (72); the values of the remaining useful life ofthe cutting performing a similar process is predicted by estimating themachine tool state via the first-order Markov process model defined by asecond order polynomial model of:Xk+a _(k) t _(k) +b _(k) t _(k) ² +c _(k) =X _(k−1)+(a _(k)+2bt_(k−1))Δt ₊ b _(k) Δt ²

wherein the Xk is the state of tool wear, tk is the time at step K, anda_(k), b_(k), and c_(k) are the parameters of the second orderpolynomial model (73); optionally, the information on the remaininguseful life can be sent to a manufacturing shop floor (74).

Process Anomaly Detection Across Machines

Anomaly detection is an important aspect for a self-aware system. Ananomaly is an exception or deviation from a typical pattern. An anomalyunder the context of a manufacturing shop is an abnormal machineprocess, and may manifest through unusual, uncustomary exceptional, orextraordinary occurrence of the tool parameters, including a rare choiceof a machine tool, unusually high or low spindle power in a machiningprocess compare to similar machining processes, unusual high or lowspindle speed in a machining process compare to similar machiningprocesses, an uncharacteristic tool path in a machining process compareto similar machining processes, as well as other behaviors. In amanufacturing shop setting, parameters for the anomaly detectionincludes spindle speed, spindle power, feed rate, tool path (X-, Y-, andZ-positions in a 3-D space), time and duration, and tool ID. Otherparameters are possible.

An anomaly under the context of a manufacturing shop does notnecessarily imply a malfunction. The rare choice of the machine tool maybe a deliberate switch of tools; the unusually high spindle speed orpower may be a stochastic event from an external power surge; and theuncharacteristic tool path may be the result of a human intervention.These intended, and occasionally unintended deviations from the normsare normal and do not necessarily result from or cause machinemalfunctions. These events may be labeled, annotated, accounted for, andexcluded from further analysis.

On the other hand, an anomaly under the context of a manufacturing shopmay well result from, and further cause a malfunction of a machine. Therare choice of the machine tool may be a mistake; the unusually highspindle speed or power may be a result of a damaged cutting edge, and islikely to further damage the cutting edge; and the uncharacteristic toolpath may results from a wrong machine setting or unintended obstacle inthe tool path, either of which will likely result in malfunction.

Thus, anomaly detection of the machining processes, when properlycarried out, can reflect underlying problems of the machines and raisean early warning, contributing to the self-awareness of the machine. Theanomaly detection may rely on the same sets of operational data used bythe tool degradation assessment and prediction as described supra, butthe thrusts of the analyses are different. The former focuses oncounting outliers, whereas the latter focuses on evaluating trends. Theanomaly detection does not require a specific model of failure, as aresult is widely applicable and easily scalable. Finally, the accuracyand efficiency of early warning and objective advice in themanufacturing shop floor setting will be greatly improved by combiningboth the anomaly detection and the degradation assessment and predictionusing the trend.

In one embodiment, a method for detecting an anomaly of a machine toolis provided. A stream of operational data comprising positionalparameters and movement parameters of machining processes performed bythe machine tool are collected. One or more outliers from the stream ofthe operational data are detected by a plurality of anomaly detectionalgorithms that are based on different failure models and assumptions.The number of the outliers present in the anomaly detection methods isdetermined. A threshold of the outliers based on the impact on workflowis selected and may adjusted. A notification can be sent when the numberof the outliers exceeds the threshold.

In a further embodiment, a method for detecting an anomaly of a machinetool is provided. A stream of operational data comprising positionalparameters and movement parameters of machining processes performed bythe machine tool are collected. The distribution patterns of theoperational data are characterized by multiple analytical methods. Newoperational data is collected and compared to the distribution patternsfrom the multiple analytical methods. The presence of one or moreoutliers against the distribution patterns of the analytical methods aredetermined. The number of the outliers are compared to a pre-determinedthreshold. Notice is sent when the number exceeds the threshold. Theanalytical methods includes self-organizing maps (SOMs), multivariateregression, and robust Mahalanobis distance, respectively.

Self-Organizing Maps (SOMs)

SOMs are a way to organize an incoming stream of data into a grid ofcells where similar data tend to end up in the same or nearby cells.Typically, a Euclidean distance metric is used when assigning the data.As data accumulates, some cells may become very dense with the assigneddata spots and is likely representative of a typical behavior or usageof a machine. Occasionally, a data instance, due to its unusual value,gets assigned to a sparsely populated cell, which indicates a deviationfrom the typical behavior or usage. These data instances may also bereferred to as outliers. It is possible to label or annotate an outlieras a desirable, intended, or planned event, thus removing the outlierfrom the consideration or estimation of malfunction.

SOMs may be 2, 3 or high-dimensional depending the numbers of parametersor variables. Typically, one parameter correlates to one dimension. Inone embodiment, spindle speed, spindle power, feed rate, X-, Y-, andZ-positions in a 3-D space, and tool ID are used for plotting7-dimensional SOMs of tool processes of a machine tool.

Although the cells or grids start out with even size, they may be warpedbased on the distribution or density of the data points within thecells, to increase the resolution in spaces of high density where themajority of the data points reside. Furthermore, the Voronoi partitionmay be applied to the cells, so that each data point within a partitionis assigned to the node associated with it.

In one embodiment, operational data of a machine, including spindlespeed, spindle power, feed rate, tool path (X-, Y-, and Z-positions in a3-D space), time, and tool ID, are used in SOM to detect outliers. FIG.7 is flow diagram showing a computer-implemented method for detectingone or more outliers from the stream of the operational data by a methodof self-organizing maps. A stream of operational data comprisingpositional parameters and movement parameters of a machine tool arecollected (81). A multidimensional space with grid of cells wasgenerated by assigning each parameter to one axis of the space (82).Each datum is placed onto a cell to display as a spot based on the valueof the datum (83). The cells are warped based on the numbers of thespots nearby to increase resolution in areas of high density of spots(84). Spot densities in the vicinity of every spot are estimated (85).If a datum spot is located in an area where the spot density is lowerthan a threshold, the datum is assigned as an outlier (86).

Multivariate Regression

The operational parameters collected during a machining process areinter-connected. For example, in a controlled system such as a CNCmachine, the high level requirements, such as a tool path, aretranslated into low level specifications, such as feed rate and spindlespeed, which are then met by control inputs (e.g. spindle power). Thuscorrelations tend to exist between the parameters, such as a high powerusage is correlated to a high spindle speed. These relationships betweenthe different parameters can be ascertained to provide a warning when anew data instance exhibit a significant departure from the ascertainrelationship.

The relationship between the parameters may be ascertained through amultivariate regression analysis. For the sake of convenience, eachparameter, or its derivative, is a variable. Once the relationshipsamong the parameters, or variables, are established, the value of adependent variable can be estimated from the values of independentvariables. Thus, a prediction interval may be obtained for a new datapoint.

In one embodiment, the parameters, or the variables, collected for themultivariate regression analysis includes spindle power, spindle speed,feed rate, tool ID, duration, and distance traveled by the tool. Thetool ID is a categorical variable and the others are numericalvariables. Instead of the entire tool path, intervals of the datacollection process are sampled. Thus instead of analyzing an entire toolpath, the distance traveled by the tool during a sampling instance areused. This combination provides sufficient resolution and predictivepower without requiring unrealistically high computing power. Othercombinations of parameters are possible.

In a further embodiment, a quantile regression forests analysis is usedbecause we found this method provides an ideal fit to the scenario ofmanufacturing shop setting. Let Q_(α)be defined asQ _(α)(x)=inf{P(Y≤y|X=x)≥α}  (7)

Then Q_(α)represents the α-quantile for the conditional distribution ofa variable Y conditioned on a vector variable X. If Y is the variablebeing predicted (independent variable) then Q_(α)defines its α-quantileconditioned on the independent variable X

In a still further embodiment, the independent variable Y, or predictivevariable, is the spindle power; and the dependent variables X, or theprediction variables are the spindle speed, feed rate, tool ID,duration, and distance traveled by the machine tool. In a yet furtherembodiment, a prediction interval of

[Q_(0.025), Q_(0.975)] is used. A wilder range of prediction intervalincreases the accuracy but reduces the sensitivity of the detectionmethod. FIG. 8 is flow diagram showing a computer-implemented method fordetecting one or more outliers from the stream of the operational databy a method of multivariate regression in accordance with oneembodiment. Incoming data on the spindle power, spindle speed, feedrate, distance traveled, duration, and tool ID for a machining processare sampled from a plurality of machining processes (91). The spindlepower is assigned as a dependent variable and the rest of the parametersas independent variables (92). The predicting interval for the spindlepower is calculated based on a quantile regression forests analysis asdefined as:Q _(α)(x)=inf{P(Y≤y|X=x)≥α}wherein Q_(α)represents the α-quantile for the spindle power conditionedon a vector variable X, which includes the spindle speed, feed rate,distance traveled, duration, and tool ID (93). A new operational datumis obtained (94). The new operational data is assigned as an outlier ifthe value of the spindle power lies outside the prediction intervalobtained from the quantile regression forests analysis (95).

An outlier refers to a data point that falls outside the predictioninterval. A warning may be sent when a new data point falls outside theprediction interval. The occurrence of one or more outliers may also becombined with other analysis to provide a refined warning.

Robust Mahalanobis Distance

In the Mahalanobis distance analysis, outliers are data points that aresamples from a different distribution rather than extreme values of themultivariate normal distribution. Thus a cutoff point is not needed forlabeling a point as outlier. The Mahalanobis distance analysis requiressamples from a multivariate normal distribution, a condition notsatisfied when looking into the operational parameters. However, thenormality distribution requirement can be satisfied by transforming theoperational data into the principal component space and looking for theoutliers in the space spanned by the top few principal components. Sinceprincipal components are aligned with directions of maximal variance,the outliers are easier to spot. Also, by looking in the reduced spaceof the top principal components, the signal to noise ratio is increased.

In one embodiment, a normalization procedure is performed to make theEuclidean distance in the principal component space equivalent to theMahalanobis distance in the original space by projecting the originalfeature space to the reduced space of the top principal components usinga transformation matrix which converts the data to the directions of thetop Eigen vectors. Outliers are measured based on its Mahalanobisdistance in the absence of any distributional assumptions by checkingwhether the Euclidean distance is within a predefined threshold in theprincipal components space. FIG. 9 is flow diagram showing acomputer-implemented method for detecting one or more outliers from thestream of the operational data by a method of robust Mahalanobisdistance in accordance with one embodiment. Incoming data aretransformed into their principle components (101). At least twoprincipal components according to the size of a variance of theprincipal components are selected to serve as axes of principlecomponent space (102). The values of the principal components arenormalized to make the Euclidean distance in the principal componentspace equivalent to the Mahalanobis distance in the original space(103). Finally, an outlier is detected based on the Mahalanobis distancein the principal components space, when the Euclidean distance of theprinciple components of the incoming data exceeds a predefined thresholdin the principal components space (104).

Ensemble of the Outlier Detection Methods

The three outlier detection approaches as described supra, e.g.,self-organizing maps, multivariate regression, and robust Mahalanobisdistance, make different assumptions and have different strengths andweaknesses. Therefore, combining them into an ensemble that can raiseflags based on some predetermined policy can be more advantageous thanusing the approaches separately. For example, if the cost of a machinefailure is very high, the ensemble may flag a data instance as anoutlier if any member of the ensemble determines the data instance to bean outlier. Such a policy is termed an “OR Policy.” Alternatively, ifthe cost of a needless disruption of workflow outweighs the cost of themachine failure, the ensemble may flag a data instance as an outlieronly if all members of the ensemble agree. Such a policy is termed an“AND policy.” In yet another scenario, the cost of a workflow disruptionand the cost of a machine failure are in balance, a good policy might befor the ensemble to flag a data instance as an outlier if a largefraction of the ensemble members agree. Such a policy is termed a“MAJORITY policy.” Finally, three outlier detection approaches,individually or in combination, may be combined with other outlierprediction method for warning purposes.

Shop Floor Planning Recommendation through Comparing Usage andPerformance Between Machines

Another level of manufacturing shop floor planning involves comparingone machine's usage and performance to another. The information can beused to avoid damage due to unintentionally excessive usage byrescheduling the machining tasks.

Typically, a machine's bearing life, expressed in term of hours ofoperation, is adversely proportional to (load³*rpm). Comparing thespindle lodes weighed by the time spent at the various spindle speedsmay provide an estimate of relative spindle damage between two machines.In one embodiment, information on spindle usage, including spindle speedand load over time are used to estimate spindle damage. A histogram ofthe spindle loads weighted by the time spent at various spindle speedsis plotted and compared for two machines, which provides a relativeestimate of remaining useful life of the spindle bearings. Thisinformation is fed back to a scheduling system depending on the shop'smaintenance policy. For example, if all machines will be taken downaround the same time for service, the machine with less spindle damagemay be scheduled for more jobs until the spindle damages are balancedout prior to the service.

The aggregate axes traverse provides another measure of wear on variousaxes in a machine. In a further embodiment, information on aggregateaxes traverse are used to provide an estimate of the way damage. Theaggregate axes traverses are compared between two machines. If any axistravels beyond a threshold that is greater than twice of a comparablemachine in the same time frame, part switching between the machines isrecommended to balance. In addition, commanded position or the tool pathof the machines are obtained. The aggregate axes traverses are comparedto the commanded position or the tool path to discovery an error inposition. Similar analysis can be employed to balance travel of variousdrive axes by shifting parts appropriately. These include rotating thefixtures based on current state and scheduled tool paths.

Warnings or recommendations based upon the usage and performancecomparison between machines may be sent to manufacturing shopsupervisors, operators, maintenance crews, or other related personnel.Measures can be subsequently taken to address the imbalance of themachine usage and wear in accordance to the objective estimate ofmachine wear and damage accumulation. As a result, manufacturing shopcan move from a scheduled maintenance to a condition-based maintenancethat more closely reflects the damage accumulation.

While the invention has been particularly shown and described asreferenced to the embodiments thereof, those skilled in the art willunderstand that the foregoing and other changes in form and detail maybe made therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A computer-implemented system for detectingmachine tool wear, predicting machine tool failure, and manufacturingshop floor planning, comprising: a computing device comprising aprocessor and configured to: obtain a machine tool's operational datacomprising positional parameters and movement parameters during a timewindow; identify a plurality of machining processes of the machine toolbased upon a match of the positional parameters and the movementparameters that defines the machining processes; cluster the pluralityof the machining processes into one or more process clusters, based upona similarity of the machining processes; detect the machine tool's wearby characterizing a trend of change in a parameter from the one or moreclusters of machining processes performed by the machine tool; andpredict the machine tool's remaining useful life by extrapolating thetrend under a first-order Markov process, wherein the machine tool isreplaced based on the predicted remaining useful life.
 2. Acomputer-implemented system according to claim 1, wherein theoperational data comprise of at least one of X-axis position, Y-axisposition, Z-axis position, feed rate, spindle speed, spindle power,spindle load, and tool number.
 3. A computer-implemented systemaccording to claim 1, the computing device further configured to:determine a tool path shape in a 3-D space for the plurality of themachining processes by plotting X-, Y-, and Z- positions of the machinetool during associated with the processes; determine a movementparameter shape by plotting the spindle speed, feed rate, and time ofthe machine tool in the parameter space for the plurality of themachining processes associated with the machine tool; and determine thematch between two machining processes from the plurality of themachining processes by matching both the tool path shapes in the 3-Dspace and the movement parameter shapes in the parameter space betweentwo machining processes, via finding the smallest possible differencesbetween the matched shapes using an ICP algorithm.
 4. Acomputer-implemented system according to claim 1, the computing devicefurther configured to: cluster the machining processes based on thesimilarity of the machining processes, further comprising: obtain aninitial cluster of machining processes; calculate the difference betweenat least one of the plurality of the machining processes and the averageof the initial cluster using a T2 method of:T2=(d _(a) −{circumflex over (d)} _(a))*s ⁻¹* (d _(a) −{circumflex over(d)} _(a))′ wherein d_(a) is a measurement of the difference andcalculated by d_(a)=[d _(s),d_(p)], d_(s) is a difference between toolpath shapes in shape spaces, d_(p) is a difference between movementparameter shapes in parameter spaces, {circumflex over (d)}_(a) is themean value of the difference of the initial cluster of the processes,and s is the covariance; determine a T2_(limit) by a formula of${{T\; 2_{limit}} = {\frac{\left( {N - 1} \right)\left( {N + 1} \right)p}{N\left( {N - p} \right)}{F_{\alpha}\left( {p,{N - p}} \right)}}},$ where F _(α)(p,N−p) is the 100α% confidence level of F−distributionwith p and N−p degrees of freedom; keep the at least one process out ofthe cluster if the T2 statistics is above the T2_(limit); and assign theat least one process into the initial cluster of the processes andupdate the centroid of the cluster if the T2 statistics is below theT2_(limit).
 5. A computer-implemented system according to claim 1, thecomputing device further configured to: characterize the trend of theparameter from one of the one or more clusters through detecting aunidirectional change of the parameter via a monotonicity test asdescribed in for${{Monotonicity}(F)} = {\frac{{\#{d/{dF}}} > 0}{n - 1} - \frac{{\#{d/{dF}}} < 0}{n - 1}}$wherein F is the measurement of the parameter, d/dF is the derivative,and n is the number of measurement in a period of time; set up athreshold of F ; and provide a notification of a degradation when thethreshold is exceeded by the calculated monotonicity, wherein theparameter used for the trend detection is chosen from at least one ofthe following group: the machine tool's spindle power, the machinetool's torque, heat generated by the machine tool during the processes,and frictions of a bearing associated with the machine tool during theprocesses.
 6. A computer-implemented system according to claim 1, thecomputing device further configured to: define the first-order Markovprocess model with a second order polynomial model of:Xk=a _(k) t _(k) +b _(k) t _(k) ² +c _(k) =X _(k −1)+(a_(k)+2bt_(k−1))Δt ₊ b _(k) Δt ² wherein the Xk is the state of toolwear, t_(k) is the time at step K, and a_(k), b_(k), and C_(k) areparameters of the second order polynomial model.
 7. Acomputer-implemented system according to claim 6, the computing devicefurther configured to: notify of the remaining useful life, wherein themachine tool is a cutting tool and the parameter is the values of thespindle powers of the processes.
 8. A computer-implemented method fordetecting machine tool wear, predicting machine tool failure, andmanufacturing shop floor planning, comprising: obtaining by a computingdevice comprising a processor a machine tool's operational datacomprising positional parameters and movement parameters during a timewindow; identifying by the computing device a plurality of machiningprocesses of the machine tool based upon a match of the positionalparameters and the movement parameters that defines the machiningprocesses; clustering by the computing device the plurality of themachining processes into one or more process clusters, based upon asimilarity of the machining processes; detecting by the computing devicethe machine tool's wear by characterizing a trend of change in aparameter from the one or more clusters of machining processes performedby the machine tool; and predicting by the computing device the machinetool's remaining useful life by extrapolating the trend under afirst-order Markov process, wherein the machine tool is replaced basedon the predicted remaining useful life.
 9. A method according to claim8, wherein the operational data comprise of at least one of X-axisposition, Y-axis position, Z-axis position, feed rate, spindle speed,spindle power, spindle load, and tool number.
 10. A method according toclaim 8, further comprising: determining by the computing device thematch of the positional parameters and the movement parameters, furthercomprising the steps of: for the plurality of the machining processesassociated with the machine tool, determining a tool path shape for eachmachining process in a 3-D space by plotting X-, Y-, and Z- positions ofthe machine tool during the each machining process; for the plurality ofthe machining processes associated with the machine tool, determining amovement parameter shape by plotting the spindle speed, feed rate, andtime of the machine tool in a parameter space for the each machiningprocess; and determining the match between two machining processes fromthe plurality of the machining processes by matching both the tool pathshapes in the 3-D space and the movement parameter shapes in theparameter space between the two machining processes, via finding thesmallest possible differences between the matched shapes using an ICPalgorithm.
 11. A method according to claim 8, further comprising:clustering by the computing device the machining processes based on thesimilarity of the machining processes, further comprising the steps of:obtaining an initial cluster of machining processes; for at least one ofthe plurality of the machining processes, calculating the differencebetween the at least one process and the average of the initial clusterusing a T2 method of:T2=(d _(a) −{circumflex over (d)} _(a))*s ⁻¹*(d _(a) −{circumflex over(d)} _(a))′, wherein d_(a) is a measurement of the difference andcalculated by d _(a)=[d_(s),d _(p)], d_(s) is a difference between toolpath shapes in shape spaces, d_(p) is a difference between movementparameter shapes in parameter spaces, {circumflex over (d)}_(a) is themean value of the difference of the initial cluster of the processes,and s is the covariance; determining a T2_(limit) by a formula of${{T\; 2_{limit}} = {\frac{\left( {N - 1} \right)\left( {N + 1} \right)p}{N\left( {N - p} \right)}{F_{\alpha}\left( {p,{N - p}} \right)}}},$ where F _(α)(p,N−p) is the 100α% confidence level of F−distributionwith p and N−p degrees of freedom; keeping the at least one process outof the cluster if the T2 statistics is above the T2_(limit); andassigning the at least one process into the initial cluster of theprocesses and updating the centroid of the cluster if the T2 statisticsis below the T2_(limit).
 12. A method according to claim 8, furthercomprising: for one of the one or more clusters, characterizing by thecomputing device the trend of the parameter through detecting aunidirectional change of the parameter via a monotonicity test asdescribed in${{Monotonicity}(F)} = {\frac{{\#{d/{dF}}} > 0}{n - 1} - \frac{{\#{d/{dF}}} < 0}{n - 1}}$wherein F is the measurement of the parameter, d/dF is the derivative,and n is the number of measurement in a period of time; setting up bythe computing device a threshold of F ; and notifying by the computingdevice of a degradation when the threshold is exceeded by the calculatedmonotonicity, wherein the parameter used for the trend detection ischosen from at least one of the following group: the machine tool'sspindle power, the machine tool's torque, heat generated by the machinetool during the processes, and frictions of a bearing associated withthe machine tool during the processes.
 13. A method according to claim8, further comprising: defining by the computing device the first-orderMarkov process model with a second order polynomial model of:Xk=a _(k) t _(k) +b _(k)t_(k) ² +c _(k) =X _(k−1)+(a _(k)+2bt_(k−1))Δt+b _(k) Δt ² wherein the Xk is the state of tool wear, t_(k) isthe time at step K, and a_(k), b_(k), and c_(k) are parameters of thesecond order polynomial model.
 14. A method according to claim 13,further comprising: notifying by the computing device of the remaininguseful life, wherein the machine tool is a cutting tool and theparameter is the values of the spindle powers of the processes.
 15. Amethod according to claim 8, further comprising: providing by thecomputing device a recommendation on scheduling a machine maintenance bycomparing a performance history of the machine to another machineperforming comparable machining process, comprising: comparing ahistogram of a spindle speed measured by RPM weighted by spindle load atthe speed for the one machine to the other; and comparing the totaltraverse of across all feed axes on the one machine to the other.